1. Field of the Invention
The invention relates to a driving system for vibration conveyors of the resonance type comprising an electromagnet with a yoke.
2. Description of Related Art
In order to convey fairly small items, vibration conveyors are frequently used which are either linear or bowl-shaped. A typical example of a vibration conveyor contains a bowl with a spiral track on the inside for the items to be conveyed. The bowl is suspended on slightly slanted leaf springs which in their turn are fastened to a heavy bottom element. Together these parts form a resonant mechanical system. The bottom element is commonly supported by vibration-damped elastomer feet. On the top of the bottom element one or several electromagnets are fastened. If these are connected. to an alternating current, they will induce the bowl to vibration by the varying magnetic attraction of a yoke on the bowl, in step with the frequency of the alternating current. Normally the alternating current supplied will have a fixed frequency, as the source is the public mains. In order to obtain sufficient vibration at this frequency it is necessary to tune the resonant mechanical system to (approximately) the same frequency as that of the supplied force. In the case of a soft iron core and yoke the frequency of the force will be twice that of the mains because of electromagnetic attraction at both positive and negative going currents. The mechanical resonance frequency will, however, vary in function of a number of factors, such as temperature, ageing of the springs, and the mass of the items to be conveyed. Furthermore, the magnetic force supplied will be dependent on variations in the mains voltage. The operator must hence continuously adjust the current supply to the vibration conveyor in order to obtain stable item conveyance. In case a high item velocity is required, it is furthermore necessary to halt operations from time to time in order to retune the resonance frequency.
Furthermore, it turns out in practice that the mechanical resonance frequency depends on the oscillation amplitude, in a such a manner that the the resonance frequency decreases with increasing oscillation amplitude. If the mechanical system is tuned to a higher frequency than the driving frequency, the above relationship will cause a positive feedback of the oscillation with increasing oscillation amplitude, and a negative feedback in the case of decreasing amplitude. The outcome of this is that the vibration conveyor suddenly runs wild when more energy is fed into it, and suddenly halts when the energy supply is reduced. Furthermore, an increase of the mass of the oscillating system by loading it with heavy items will result in a lower resonance frequency, so that this will approach the drive frequency, whereby the oscillation amplitude increases, and the above mentioned phenomenon of positive feedback manifests itself. In order to obtain a stable conveying system it is hence necessary to tune the vibration conveyor resonance frequency such that it is 5-10% lower than the drive frequency which calls fore more energy input to obtain sufficient oscillation or vibration amplitude.
Even though the energy requirement is reduced because of the resonance, there is still, in large conveying plants, a requirement for an overdimensioned electrical installation due to the large reactive current caused by the large airgaps in the electromagnets. This results in either a higher payment to the electricity company or the need for investment in phase compensators. In traditional systems the amplitude of the vibrator oscillation is adjusted by means of a variable mains transformer which has to be individually adjusted for each individually tuned conveyor unit. Alternatively, a power regulation of the phase control type may be used. All these factors are considered as particular disadvantages of the known systems.
In order to partially remedy these disadvantages it has been tried to use a variable frequency to drive vibration conveyors, i.e., to tune the individual driving frequency according to the prevailing mechanical resonances. Simple frequency converters have been used, but better results are obtained by determining the amplitude and frequency of the oscillation itself by means of an accelerometer which provides signals which may be used in closed-loop control of the frequency generators. This, however, entails a complication in installation, and it must be ensured that the cable carrying the accelerometer signal is not subjected to breakage or causes microphony.
From published European Patent Application EP 0 629 568 A2 a construction is known where the vibration conveyor is driven from a voltage source which produces rectangular pulses. The frequency and pulse width of these may be varied so that the effective coil current is varied correspondingly in frequency and amplitude. Hereupon the oscillation amplitude is measured and expressed as the amplitude of the third harmonic in the coil current (in the case of piezoelectric vibrators expressed by means of the second harmonic). It has been established that there is an approximately linear relationship between the content of third harmonic and the oscillation amplitude. The amplitude of the third harmonic is used as feedback in a control loop in order that a given-amplitude may be retained. The resonance frequency is found by means of a sweep of the frequency range and locking of the drive frequency when the content of third harmonic is maximum. Henceforth, the drive frequency is kept constant until a desired oscillation cannot be maintained, even at maximum current. A new sweep is performed, and the frequency is again locked. This means that each time that items are filled into the vibration conveyor, and along with the emptying of the items, a new frequency sweep will be required. This calls for undesired pauses many times per hour.
Another known construction is described in U.S. Pat. No. 4,811,835. This case deals exclusively with a bipolar type, i.e., where the yoke is a permanent magnet. Here, the mechanical oscillation frequency will be equal to the drive frequency. If the drive voltage is sinusoidal, the drive current will have a signal superimposed which is caused by the movement and which will hence have the same frequency as the drive frequency. The phase shift of this signal will follow the phase of the oscillation in such a way that it is shifted -90.degree. with respect to the drive current and far from resonance 0.degree. or 180.degree. with respect to the drive current. At resonance it will hence contribute a phase shift in the total current while far from resonance it will only influence the amplitude of the drive current.
The patent uses this, in that the phase between current and voltage is measured continuously, and the frequency is adjusted so that the phase shift between drive current and drive voltage is minimum (i.e. the phase of the oscillation is -90.degree.). This construction is unable to determine whether the instantaneous drive frequency lies above or below the resonance frequency. Hence, the frequency is adjusted in a given direction, until the phase change since last measurement is positive, i.e., moving away from resonance. The direction is changed, and the sweep is restarted. In order to maintain a constant oscillation amplitude the measured values of current and voltage are used to calculate the power supplied. The amplitude of the drive voltage is adjusted in order that the supplied power is held constant. It is held that the feed velocity of the items has a linear dependency on the supplied power. There is hence no feedback in this control.
A third known construction is described in published European Patent Application EP 0 432 881 A1 which regards a piezoelectrically driven vibrator. The measurement signal is obtained in a similar fashion as above, but the phase is used to calculate the power uptake, and it is this which the control attemps to keep constant. Intervals of amplitude control alternate with intervals of frequency control. Thus, the amplitude is controlled at a fixed frequency so that the power uptake becomes a predetermined value, and during the subsequent frequency control, the target is maximum power uptake. This control method is very slow, and sudden loading with items will cause a long waiting period until a suitable frequency (resonance) has been found.
None of the constructions described is able to brake the oscillation actively. Braking of the oscillation by controlling a drive current in counterphase to the movement can only be attained if the phase of the movement is known continously and can control the braking current phase-locked to the movement.